The present invention relates to satellite position systems which use reference receivers and more particularly to a network of reference receivers for a satellite positioning system.
Conventional satellite positioning systems (SPS) such as the U.S. Global Positioning System (GPS) use signals from satellites to determine their position. Conventional GPS receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of GPS satellites which orbit the earth. Each satellite transmits, as part of its navigation message, both satellite positioning data as well as data on clock timing which specifies its position and clock state at certain times; this data, found in subframes 1-3 of the GPS navigation message, is often referred to as satellite clock and ephemeris data and will be referred to as satellite ephemeris data. Conventional GPS receivers typically search for and acquire GPS signals, read the navigation message from each signal to obtain satellite ephemeris data for its respective satellite, determine pseudoranges to these satellites, and compute the location of the GPS receiver from the pseudoranges and satellite ephemeris data from the satellites.
Improved position accuracy can be obtained by using a well known and conventional technique referred to as differential GPS. With conventional differential GPS, a single differential reference station broadcasts differential GPS corrections to users in a local region. Thus, there are typically three major components of a conventional differential GPS system. The first component is a reference station at a known location with a GPS receiver at a known location which is usually capable of observing all satellites in view and optionally with software at the reference station, which could be imbedded in the GPS receiver, to compute the pseudorange corrections and to code them for specific broadcast format. Another component is a radio link to transmit the differential corrections in real time to mobile GPS receivers. The third component is the mobile GPS receiver which also includes a receiver for receiving the differential corrections broadcast from the reference station.
The differential GPS corrections are used by the mobile GPS receivers in a conventional manner to correct the pseudorange data which is obtained by computing the relative times of arrival of signals of GPS signals transmitted from the GPS satellites. Conventional differential GPS does not have to operate in real time or provide corrections to the mobile GPS receiver, although this is often the case. There are many improvements on differential GPS which are described in both patent and non patent literature. These various improvements concentrate on the differential correction computation and application algorithms as well as methods of delivering the differential corrections. The differential corrections are for the most part in the measurement domain (pseudorange, accumulated delta-range, and range-rate error estimates).
Conventional differential GPS offers significant position accuracy improvement if both the reference receiver and the participating mobile GPS receiver are in close proximity to each other. However, the accuracy improvement from differential GPS degrades as the separation distance between the two receivers increases. One solution to rectify this degrading of accuracy is to provide a network of GPS reference receivers which are dispersed over a geographical area to provide area coverage which coincides with the area in which the mobile GPS receivers may operate such that they tend to see the same set of satellites. In this instance, a mobile GPS receiver may pick up differential corrections from more than one differential reference station, and the mobile GPS receiver may select those differential corrections for satellites in view based upon the relative proximity between the mobile GPS receiver and the two or more reference stations. The use of multiple reference stations in a differential GPS system is sometimes referred to as wide-area differential GPS (WADGPS).
A further form of a WADGPS reference system includes a network of GPS reference receivers and a master station which is in communication with the reference stations to receive their measurements and compute a merged set of ephemeris and clock correction estimates for each GPS satellite observed by the reference stations. This master station can then provide through a transmitter a differential GPS message with corrections applicable over an extended range. Examples of such wide-area differential GPS reference systems include those described in U.S. Pat. Nos. 5,323,322 and 5,621,646.
Independent of the coverage of the particular differential reference system, the prime objective of a differential GPS system is to provide differential service which helps the mobile GPS receiver to remove errors from the GPS measurements or measurement derived solution. The GPS system errors that the network attempts to remove is a function of the number of reference stations, their spatial placement, and the sophistication of the algorithms implemented at the central processing facility. The secondary function of the differential networks is to provide integrity and reliability to the differential service by performing various checks in the measurement and the state space domains.
While the foregoing systems provide improved accuracy to mobile GPS receivers, those systems are not compatible with a client/server GPS architecture in which a mobile GPS receiver functions as a client system and provides pseudorange measurements to a remotely positioned location server which completes the calculations for the position solution by using the pseudoranges obtained from the mobile GPS receiver and by using ephemeris data. The present invention provides an improved method and apparatus allowing flexibility in the positioning of location servers and also provides for improved efficiency and cost in a client/server system.